Apparatus and process for nickel plating and sealing

ABSTRACT

A pre-treatment process surface preparation of workpieces for subsequent nickel coating is provided including a thermal process to remove residue, grit blasting for surface roughening, such as with garnet, and rust avoidance by submergence in hot ammonia. Use of expensive tank materials for electroless nickel coating are avoided through the provision of replaceable liners optionally protected from the subject workpieces using movable wall panels. Further, tank heating is effectively provided using industrial hot water heaters and an indirect water/nickel solution heat exchanger, obviating the need for risky and regulated heat sources. In a final option, a fluoropolymer sealer is provided over the nickel coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefits under 35 U.S.C. 119(e) of the U.S. Provisional Application Ser. No. 61/771,171, filed on Mar. 1, 2013, the subject matter of which is incorporated fully herein by reference.

FIELD

This subject matter of this disclosure relates generally to apparatus, process and systems for effectively corrosion-proofing metal workpieces, more particularly cleaning and plating a steel workpieces with nickel and protective coatings.

BACKGROUND

Nickel plating is a process that deposits a thin layer of nickel onto an underlying metal, often steel. Some of the benefits of nickel plating include increased resistance to corrosion or rust, improved resistance to wear, strength and improved ductility. Workpieces are typically manufactured of vulnerable and inexpensive materials. Nickel plating is often used in the automotive and oil and gas industries to protect such materials from corrosion caused by CO₂ and H₂S.

Two different methods may be used to add nickel plating to a workpiece. The first is electrolytic, also called “galvanic”. The second is purely a chemical process known as electroless which relies upon a chemical reaction to apply the layer of nickel.

Before performing electroless nickel plating, the material to be plated is cleaned by a series of chemicals, known as the pre-treatment process. Chemical cleaning is the standard cleaning process in the plating industry. Chemical cleaning is only an effective as the cleaning solution. The success of the cleaning has a direct impact on the adhesion of the nickel coatings. Workpieces that are not cleaned properly will have poor adhesion. In particular, failure to remove unwanted residue or “soils” from the workpiece's surface result in poor plating. Each pre-treatment chemical treatment is therefore followed by several water rinses to remove chemicals that may adhere to the surface. De-greasing removes oils from surfaces, whereas acid cleaning removes scaling. The cleaning bath may require frequent replacement at a high cost. Further, after the cleaning process is complete, the workpiece is exposed to a Hydrochloric acid (HCl) pre-dip prior to plating. However HCl pre-dip tends to leave a rust film or “Rose Bloom” on the surface of the workpiece which also negatively affects the adhesion of the nickel coating.

Once the workpiece has been prepared, it is placed in an alloy tank that is passivated by nitric acid for plating. The tanks are manufactured of exotic metals resistant to the chemicals and plating effects. The tanks are typically made of stainless steel or alloys such as Inconel® (International Nickel Company, Inc.). The chemical bath or fluid in the tanks is typically heated by electric elements or boilers coupled with steam heat exchangers. Electric elements are expensive, have a limited size of tank they can heat and may take multiple hours to heat the fluid in the tank. If an electrical element were to be damaged, it could cause a fire and pose a safety risk to the workers. Boilers coupled with steam heat exchangers are far more efficient. However, they have specific operational and maintenance requirements which are often regulated and require certified operators with special training. Steam heat exchangers are not readily serviceable and are typically replaced when their life cycle ends. Replacing steam heat exchangers can be expensive and time consuming, with additional revenue lost during a shut down and servicing.

The newly plated workpiece is sometimes further coated with a sealer to enhance the corrosion resistance of the nickel coating. Unfortunately, most sealers wipe or peel off over time and effectively offer little to no additional protection.

There is a need for an improved nickel plating process and equipment in the coating industry.

SUMMARY

Generally, a pre-treatment process prepares workpieces, such as iron-bearing workpieces, for subsequent electroless nickel coating. The pre-treatment process comprises thermal heat soaking and garnet grit blasting. Further, the workpiece can be submerged in hot ammonia, all of the above pre-treatment steps resulting in minimal residue and actors adverse to subsequent nickel plating operations.

Further, use of expensive tank materials for electroless nickel coating can be avoided through the use of mere carbon steel tanks protected by replaceable plastic liners. Somewhat fragile liners can be optionally protected from the subject workpieces using movable panels suspended strategically about the tank walls. Further, tank heating is effectively provided using industrial hot water heaters and an indirect water/nickel solution heat exchanger, obviating the need for risky and regulated heat sources such as electrical or steam heating. In a final option, a fluoropolymer sealer can be provided over the nickel coating.

In a broad aspect a pre-treatment method for preparing a workpiece for plating comprises heat-soaking the workpiece for thermal removal of residue therefrom, optionally at about 400° C. for about 8 hours, and blasting the residue-free workpiece with garnet grit. Further, the process can include submerging the workpiece in hot ammonia.

In another aspect, the aforementioned pre-treatment methodology is preparatory for a method of coating the workpiece with a protective coating including using electroless nickel coating. The nickel coating can be conducted in a tank containing a nickel solution, whilst heating the nickel solution to about 90° C. The nickel solution can be heated by circulating the nickel solution through an indirect heat exchanger; and circulating hot water to the indirect heat exchanger for heating the nickel solution. Further, the nickel coated workpiece can be further protected with a polmer sealer, such as a fluoropolymer sealer.

In another aspect, the electroless nickel coating can be conducted in apparatus comprising a carbon steel tank; and a plastic liner for storing a nickel solution separated from the carbon steel tank. The apparatus can also comprise one or more bumpers for arrangement between the tank and the workpiece for protecting the liner from damage. The nickel solution can be heated using a heating system further comprising: an indirect heat exchanger; a hot water heater; a hot water circulation loop between the hot water heater and the indirect heat exchanger; and a nickel solution circulation loop between the tank and the indirect heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a flow chart of various embodiments of workpiece surface preparation including thermal cleaning, grit blasting, and hot ammonia pre-dip;

FIG. 1B is a flow chart of various embodiments of workpiece surface plating and sealing including nickel coating and polymer sealing;

FIG. 2 is an end cross-sectional view of an ENC tank having a liner, a plurality of safety bumpers and a workpiece such as a valve body being lowered by rack into the tank;

FIG. 3A is an embodiment of an electroless nickel coating (ENC) or plating embodiment illustrating hot water heaters, hot water loops, an indirect heat exchanger and ENC tank;

FIG. 3B is a schematic overview of the heating system according to the ENC embodiment of FIG. 3A including an optional ENC tank filter; and

FIG. 4 illustrates scanning electron microscope or SEM images of a first iron workpiece coated with nickel and a second iron workpiece coated with nickel and then coated with a subsequent fluoroploymer sealer according to embodiments disclosed herein.

DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with one aspect of the disclosure, there is provided a cleaning process, a nickel coating and sealing process, and methods of providing protective coatings upon parts and components, referred to generically herein as workpieces, for improving corrosion resistance of said workpieces to toxic environments that can cause premature wear or failure in industries including oil and gas and automotive. The coating process includes preparation of the surface of the workpiece, electroless nickel coating or plating in a chemical bath, and sealing the plated workpiece with a sealant.

Generally, workpieces to be protected are prepared using a thermal process to remove substantially all residues adverse to subsequent and relevant coating processes referred to herein. Further, the workpiece surface is further treated to improve coatings adhesion, more particularly using a grit blasting. Iron-bearing workpieces can further be advantageously pre-treated using a hot ammonia bath. Embodiments of nickel-plating tanks are provided herein having advantages in ease of construction, scalability, and heating. Low cost tanks can be employed having replaceable liners secured thereto. Heating is provided using hot water heaters, obviating the need for troublesome and expensive electrical or steam boilers. In a final option, a fluoropolymer sealer can be applied which is particularly effective in adhering to the nickel.

Herein, workpieces to be coated typical include valves and other workpieces manufactured of base materials vulnerable to corrosion including steel.

Surface Preparation

With reference to FIG. 1A, a pre-treatment surface preparation process is useful prior to plating and maximizes the effectiveness of nickel coating.

A first step in an embodiment of the surface preparation process is to clean a workpiece 10 as shown in Blocks 100, 110 and 120.

Thermal Cleaning

At Block 100, the workpiece 10 is thermally cleaned in an oven 20. Applicant has found that, a majority of the time, thermal cleaning is most effective for removing foreign substances from the workpiece 10 for ensuring sufficient adhesion of materials during subsequent plating processes. The oven 20 is heated to about 400° C. and the workpiece 10 is heat-soaked for a duration of about eight hours. Thermal cleaning leaves no chemical residue on the surface of the workpiece, maximizing adhesion of subsequent nickel coating in the plating process.

Grit Blasting

At Block 110, a next step in preparing the surface of the workpiece 10 for plating is grit blasting. Most conventional abrasive grit such as aluminum oxide, steel shot, or silica grits provide minimal surface roughening and grit residue is damaging to subsequent nickel coating chemicals and processing. Applicant has determined that a garnet grit is suitable for pre-nickel plating preparation without the aforementioned disadvantages. In one embodiment, garnet abrasive grit is used in the blasting process for creating a uniform surface profile virtually free of grit embedment, and providing an excellent roughened surface for coating adhesion.

A source 30 of grit 32 is provided for delivery to blasting apparatus 34 for delivery under pressure to the workpiece 10. An effective size for the garnet grit or abrasive particles is about 120 mesh (Tyler Equivalent). The pressure ranges of the blasting apparatus 34 can range from about 60 psi to about 80 psi with a preferred pressure of about 75 psi. The grit blasting is done to a National Association of Corrosion Engineers (NACE) specification #1 standard.

Hot Ammonia Bath

At Block 120, once the grit blasting is complete, the workpiece 10 is placed into a hot ammonia bath 40 for the final stage of the surface preparation. The ammonia bath has a concentration ranging from about 0.06% to about 0.10% and is heated to a temperature range of about 60° C. to about 75° C. The workpiece 10 is submerged in the bath 40 for about 0.5 hours to about 1.5 hours as appropriate depending on the thickness and volume of the workpiece 10. The hot ammonia bath 40 is used to lessen or eliminate any potential rust film or “rose bloom” that could otherwise occur on the workpiece's surface.

At Block 130, the workpiece 10 is removed from the ammonia bath 40 and ready for plating.

Plating

Once the workpiece 10 has been prepared, electroless nickel plating (ENC) can be effectively implemented. ENC is known in the art and comprises deposition of a nickel-phosphorous or nickel-boron alloy onto the workpiece. A known reducing agent reacts with metal ions in a metal workpiece to deposit the coating. Herein, the plating process is known however the tank and heating process are improved over the known apparatus and methodologies.

A nickel super alloy coating result that imparts superior corrosion resistance and added wear resistance to the workpiece. The coating is a grain-free amorphous structure with excellent barrier corrosion protection. The morphology of the coating is similar to that of a metallic glass coating, the absence of a well-defined crystal structure avoiding intergranular corrosion. Typically, in a nickel-phosphorous system, the process results in coating having a nickel metal content (wt %) of about 87.5%-89.5% and a phosphorus content (wt %) of about 10.5% to 12.0% with traces of other elements.

Turning to Block 140, the workpiece 10 is lowered into an ENC tank. The tank includes a liner 52 and a heated nickel coating solution 54. The coating solution 54 is heated through an indirect heat exchanger 56. An industrial hot water heater 58 provides the heat source for the heat exchanger 56. The workpiece is plated with nickel forming a nickel plated workpiece 10N.

At Block 150, the nickel-plated workpiece 10N is introduced to a sealer tank 60 for submergence of the workpiece 10NS or a spray booth for larger workpieces 10N. A sealer 62 is applied forming a sealed, nickel-plated workpiece 10NS.

In an embodiment, a fluoropolymer sealer 62 is used to enhance corrosion resistance of the nickel coating. The advantage of the fluoropolymer sealer is it does not wipe off or peel off as prior art sealers or dyes do. Applicant believes this is accomplished by the nature of the chemical bonding between the sealer and the nickel coating.

With reference to FIGS. 1A to 3, various arrangements and configurations of the plating equipment, and operation thereof, are described in greater detail.

Coating Equipment

With reference to FIG. 2, upon completion of the surface preparation, the workpieces 10 are treated in the nickel plating tank 50. The workpieces can be arranged and supported in a rack or basket for processing in tank 50. In the present embodiment, ENC treating tanks 50 can up to 35 feet long for long tubular workpieces. Custom tanks can be made that can accommodate a larger workpieces, Applicant regularly provides tanks 50 having dimensions in the order of 6 feet wide by 10 feet long by 6 feet deep.

In the prior art, stainless steel tanks are typically passivated by nitric acid to maximize the natural anti-corrosion properties of the stainless steel. Continuous operation requires alternating operation of dual tanks sitting side-by-side for every operational set up. Typically one tank is in operation and the other is being passivated with nitric acid. The time required to passivate one tank is several days.

Instead, in embodiments disclosed herein, mere carbon steel tanks can be used, implementing a replaceable plastic liner just slightly smaller than the tank itself. Accordingly a tank 50 has walls 70, made from a less expensive material such as carbon steel, can be used in combination with the inexpensive liner 52. Accordingly, tanks of virtually any dimensions can be readily manufactured or modified for various configurations at a fraction of the cost of tanks manufactured of more expensive, exotic corrosion resistant materials. Thus, the nickel solution can be stored in the carbon steel tank, the carbon steel tank 50 incorporating the plastic liner 52 for storing the nickel solution separated from the carbon steel tank.

In an embodiment, dual side-by-side tanks can still be employed for alternating use; one tank being used for plating while the alternate tank's polypropylene liners are changed out, the time and cost to replace liners 52 being substantially less than that for prior stainless steel tanks.

The carbon steel tank 50 comprises an open top 72 having a top edge 74 about a perimeter of the enclosing walls 70. The liner 52 is positioned inside the tank 50 for forming a liquid containment and is stretched out over the top edge 74, protecting the material of the tank walls 70. The liner can be disposable or cleaned.

Workpieces 10 are lowered into the tank 70 in a basket, from a rack 76 or other apparatus. The liner 52 is at risk of damage from the suspended workpieces. Accordingly, as shown, one or more wall panels or safety bumpers 77 formed of sheet materials are provided, such as those made from a durable material such as Polytetrafluoroethylene (PTFE) or Teflon® (registered trademark of DuPont) are movable and are installed at various points along the perimeter of the tank 50 to protect the liner 52 from damaging contact with workpieces. One means for retaining the safety bumpers 77 is to incorporate a “U” shaped hook or holder 78 at an upper end for engaging the top edge 74 of the tank, a downwardly depending panel portion 79 sandwiching the liner against the wall 70. The panel portion 79 of the bumper extends toward the bottom of the tank adjacent the interior of the perimeter wall 70 for protecting the liner. A plurality of safety bumpers 77 can be placed and moved anywhere about the perimeter of the tank as necessary. for protecting the liner from damage.

The bumpers 77 can be strategically arranged placed between the tank and the workpiece dependent upon workpiece ingress and egress locations or be implemented along substantially the entirety of the tank walls 70. Overhead fluid lines can strategically access the tanks fluid contents spaced inwardly from the liner 52 or aligned with bumpers 77 to minimize risk of liner damage.

In one embodiment, the liner 52 is a disposable polypropylene material. When the bath needs to be emptied the polypropylene liner 52 can be reused or simply discarded. The liner can be continuous into and out of which fluids are passed from overhead fluid lines. Alternatively, the liner can be manufactured with an integrated outlet, and optionally an integrated fluid inlet for in-tank fluid access.

In an embodiment, an advantage of using a disposable liner 52 is that only one operational tank could be required as the change out time is minimal, being four hours or less, minimally impacting operations. Also, capital costs for using a carbon steel material for the tank 50 with a liner 52 are typically less than one third the cost of using a stainless steel tank. The polypropylene liners 52 can be custom made to fit any size tank 50. Typically, one polypropylene liner 52 can be used for multiple cycles, typically two bath cycles.

Heating the Coating Tanks

With reference to FIGS. 1B, 3A and 3B, a heating system for the nickel plating process is provided. A nickel coating solution used in plating processes is maintained, within the ENC tank, at a temperature of about 90° C. Industrial hot water heaters 58 with large capacity heat exchangers 56 are utilized for providing the heat source required for the nickel plating process. Such hot water heaters are easy to operate and maintain and do not require specially certified workers to operate as do steam heat sources.

The industrial hot water heaters indirectly heat the solution. Large capacity plate-type heat exchanges 56 are serviceable and easy to maintain compared to electric elements and boilers. The cost for heating the system is less than that associated with electric elements and steam boilers commonly used in the industry.

The hot water heater, such as a boiler, heats water and circulates the hot water through a main loop using a main lop pump. When heat is required for electroless nickel coating tank, a primary pump pulls water from the main loop and circulates the hot water to a plate heat exchanger. Similarly, a recirculation pump circulates nickel solution through the same heat exchanger for heating the solution. The heat demand by one or more heat exchangers and the resulting heat loss from the main loop is reheated by the hot water heater.

In more detail, and having reference to FIGS. 3A and 3B, a main heated water loop 80 acts as an intermediary stage between the hot water heater 58 and heat exchanger 56. The heat exchanger 56 has a heat-providing side and a heat-receiving side, for heating and maintaining the nickel coating solution bath at a temperature of about 90° C. Thus, the heating system comprises the indirect heat exchanger 56, the hot water heater 58; hot water circulation loop 80 between the hot water heater and the indirect heat exchanger; and a nickel solution circulation loop 91 between the tank 50 and the indirect heat exchanger 56.

Hot water is provided to the main loop 80 by a hot water boiler pump 82. A main hot water loop pump 84 circulates the hot water through the main loop 80. The main loop can provide heat for one or more of the process needs such as a heat exchanger serving the hot ammonia bath.

A temperature sensing device, such as a thermocouple 86, monitors the temperature of the nickel coating solution bath in tank 50. When the bath temperature falls below a predetermined threshold temperature, the main heated water is routed through the heat exchanger 56 by a primary pump 88.

With regards to the heat receiving side, a recirculation pump 90 is provided for circulating the nickel coating solution along the nickel solution circulation loop 91 to and from the treating tank 50. The solution receives heat from the heated water at the heat exchanger 56 and is circulated back into the treating tank 50. The solution is circulated through a filter 92 with a filter pump 94 for maintaining a desired quality of the nickel coating solution for coating the workpiece, and maximizing the solution life.

Upon completion of the nickel coating process the workpiece 10 is inspected for quality assurance purposes and then sent to the sealer tank.

Sealer

Returning to FIG. 1B, a fluoropolymer sealer is used to enhance corrosion resistance of the nickel coating, the effectiveness of which is believed effective due to a chemical bonding between the PTFE and the nickel. A particularly effective fluoropolymer sealer for application over nickel coatings, such as for excellent chemical corrosion resistance for CO₂, H₂S, brine and chlorides, is a polytetrafluoroethylene (PTFE). A suitable PTFE is Teflon® (a registered trademark of DuPont) such as an acqueous dispersion of PTFE TE-3893 fluoropolymer resin made by DuPont™. As set forth in DuPont's product information for PTFE TE-3893 fluoropolymer resin, the product is a milky white liquid containing typically 60% (total wt %) of 0.05 to 0.5 um of PTFE particles suspended in water and may contain 6% (wt % of PTFE) of a non-ionic wetting agent and stabilizer. The sealer is suitable for conventional dip or flow techniques.

The sealer tank 60, which is typically about 6 feet long by 35 inches wide by 4 feet deep is used to submerse the nickel-plated workpiece 10N in the PTFE resin for about 5 minutes at room temperature (approximately 20° C. to about 25° C.). If the workpiece 10N is too large to be submerged in the sealer, the sealer 62 can also be applied by spraying the nickel surface. After spraying, excess sealer 26 can be rinsed off of the workpiece 10NP. For fluoropolymer resins, excess sealer 62 can be rinsed off after about 5 minutes.

The sealer can be applied in a single coat application or multiple coats if required and is self-lubricating and non-stick. The sealer coating results in a low coefficient of friction, providing a high level of release and non-stick properties.

The advantage of the fluoropolymer sealer is it does not wipe off or peel off as prior art sealers or dyes do. Applicant believes this is accomplished by the nature of the chemical bonding between the sealer and the nickel coating.

As shown in FIG. 4, a scanning electron microscope or SEM of a cross-section of two iron (Fe) workpiece surfaces are illustrated, both having nickel (Ni) coating, and one workpiece 10NS having a PTFE sealer and the other workpiece 10N without. 

1. A method for preparing a workpiece for plating comprising: heat-soaking the workpiece for thermal removal of residue therefrom; and blasting the residue-free workpiece with garnet grit.
 2. The method of claim 1 wherein the heat-soaking further comprises heating the workpiece at about 400° C. for about 8 hours.
 3. The method of claim 1 wherein the garnet grit is about 120 mesh.
 4. The method of claim 1 further comprising submerging the workpiece in hot ammonia.
 5. The method of claim 4 wherein the hot ammonia has a concentration of about 0.06 to 0.10% in water.
 6. The method of claim 5 wherein the workpiece is submerged in the hot ammonia at temperatures of about 60° C. to about 75° C. for about 0.5 to about 1.5 hours.
 7. The method of claim 1 wherein the workpiece is iron bearing.
 8. A method of protective coating a workpiece comprising: preparing the workpiece using the method of claim 1; and nickel coating the workpiece using electroless nickel coating.
 9. The method of claim 8 wherein the nickel coating is performed in a tank containing a nickel solution, further comprising heating the nickel solution to about 90° C.
 10. The method of claim 9 further comprising: circulating the nickel solution through an indirect heat exchanger; and circulating hot water to the indirect heat exchanger for heating the nickel solution.
 11. The method of claim 9 further comprising recirculating the nickel solution through a filter.
 12. The method of claim 8 comprising applying a polymer sealer over the nickel coating.
 13. The method of claim 8 comprising applying a fluoropolymer sealer over the nickel coating.
 14. The method of claim 8 comprising applying a sealer comprising a dispersion of a PTFE fluoropolymer.
 15. The method of claim 9 further comprising: storing the nickel solution in a carbon steel tank. lining the carbon steel tank with a plastic liner for storing the nickel solution separated from the carbon steel tank.
 16. The method of claim 15 further comprising: extending the liner over a top edge of the tank; and arranging one or more bumpers between the tank and the workpiece for protecting the liner from damage.
 17. The method of claim 16 further comprising: providing a plurality of the bumpers; and arranging one or more of the plurality of bumpers between the tank and the workpiece.
 18. Apparatus for electroless nickel plating of a workpiece comprising: a carbon steel tank; and a plastic liner for storing a nickel solution separated from the carbon steel tank.
 19. The apparatus of claim 18 further comprising one or more bumpers for arrangement between the tank and the workpiece for protecting the liner from damage.
 20. The apparatus of claim 18 further comprising: an indirect heat exchanger; a hot water heater; a hot water circulation loop between the hot water heater and the indirect heat exchanger; and a nickel solution circulation loop between the tank and the indirect heat exchanger. 